Patent Document 1 discloses a technique for reducing a transmission loss of a MCF of which a plurality of cores is made of pure silica glass, and Patent Document 2 discloses a technique for reducing the transmission loss by adding alkali metal to the core. In addition, in Patent Document 3 discloses a structure in which each single core fiber is easily separated by employing a glass in which alkali metal easily melted by an acid or the like is doped, as a glass for covering the outermost peripheral surface of a plurality of single core fibers constituting a bunch fiber. A transmission loss of the MCF manufactured by the Stack and Draw process is described in Non-Patent Document 1, and a transmission loss of the MCF manufactured by the rod-in-collapse method is described in Non-Patent Document 2.